Fuzz bug observed in CI for //xls/dslx/frontend:scanned_token_to_string_fuzz_test

https://github.com/google/xls/actions/runs/8179939932/job/22366999636

```
FAIL: //xls/dslx/frontend:scanned_token_to_string_fuzz_test (see /home/runner/.cache/bazel/_bazel_runner/5bd26e4beeff37176a5bf6fac1c974c7/execroot/com_google_xls/bazel-out/k8-opt/testlogs/xls/dslx/frontend/scanned_token_to_string_fuzz_test/test.log)
INFO: From Testing //xls/dslx/frontend:scanned_token_to_string_fuzz_test:
==================== Test output for //xls/dslx/frontend:scanned_token_to_string_fuzz_test:
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from ScanFuzzTest
[ RUN ] ScanFuzzTest.ScanningGivesErrorOrConvertsToOriginal
FUZZTEST_PRNG_SEED=0d2SWKLrxEhDRw7c_10zlZ7jRHV8K94fW0XFwJyX5oY
xls/dslx/frontend/scanned_token_to_string_fuzz_test.cc:48: Failure
Expected equality of these values:
test_module
Which is: "\"\\\\\xF9\""
reversed
Which is: "\"\\\xF9\""
Stack trace:
0x5593d312a2d6: (anonymous namespace)::ScanningGivesErrorOrConvertsToOriginal()
0x7f440445c3ae: fuzztest::internal::FuzzTestFuzzerImpl::RunOneInput()
0x7f440445f5be: fuzztest::internal::FuzzTestFuzzerImpl::RunInUnitTestMode()
0x7f4406940c1e: fuzztest::internal::GTest_TestAdaptor::TestBody()
0x7f4404e0a8fd: testing::internal::HandleExceptionsInMethodIfSupported<>()
0x7f4404e0a7be: testing::Test::Run()
0x7f4404e0bbc1: testing::TestInfo::Run()
... Google Test internal frames ...

=================================================================
=== BUG FOUND!

xls/dslx/frontend/scanned_token_to_string_fuzz_test.cc:56: Counterexample found for ScanFuzzTest.ScanningGivesErrorOrConvertsToOriginal.
The test fails with input:
argument 0: ""\\\371""

=================================================================
=== Reproducer test

TEST(ScanFuzzTest, ScanningGivesErrorOrConvertsToOriginalRegression) {
ScanningGivesErrorOrConvertsToOriginal(
"\"\\\\\371\""
);
}

=================================================================
[ FAILED ] ScanFuzzTest.ScanningGivesErrorOrConvertsToOriginal (346 ms)
[----------] 1 test from ScanFuzzTest (346 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (346 ms total)
[ PASSED ] 0 tests.
[ FAILED ] 1 test, listed below:
[ FAILED ] ScanFuzzTest.ScanningGivesErrorOrConvertsToOriginal
```